1. Field of the Invention
The present invention relates to a delay profile generator that generates a delay profile in an OFDM modulating system using scattered pilots such as ISDB-T or the like.
2. Description of the Related Art
In recent years, the OFDM (Orthogonal Frequency Division Multiplexing) modulating system has been used as a modulating system in terrestrial digital broadcasting and the like.
In the OFDM modulating system, symbols are transmitted by using plural sub-carriers (carriers) whose central frequencies are different. One symbol period is structured by a guard interval being attached to an effective symbol period. In the OFDM modulating system, as shown in FIG. 4, a portion of the effective symbol signal which is the object of demodulation in actuality is copied, and is inserted between effective symbol signals as a repeating waveform. The effects of multipath interference due to delay waves arising are thereby suppressed. The time period of this copied waveform is a guard interval (GI).
When demodulating the OFDM signal, the received OFDM signal is digitally converted by an A/D converter, the guard interval is eliminated, and the effective symbol signal is taken-out and demodulated by FFT (a Fast Fourier Transformer).
Further, in the OFDM modulating system, a scattered pilot system is used. In the scattered pilot system, pilot symbols are scattered among the data symbols in the frequency direction and in the time direction by using the amplitude or phase as the reference, the signal is modulated and transmitted and demodulation is carried out by using the pilot symbols at the receiving side by carrying out transfer path characteristic estimation.
In the transfer path estimation, a delay profile, that shows the delay time distribution of the delay wave, is generated. The delay profile is generated by extracting the pilot symbols from the results of the FFT processing and carrying out IFFT (Inverse Fast Fourier Transform) processing by using the extracted pilot symbols.
Note that, from the physical properties, the maximum time length of a delay profile that can be computed corresponds to the product of the reciprocal of the subcarrier interval at which the pilot symbols are arranged and the effective OFDM symbol length. In ISDB-T and the like, there is a symbol array such as shown in FIG. 5. Time (the OFDM symbols) is shown on the vertical axis in FIG. 5, and frequency on the horizontal axis. The black dots in FIG. 5 represent pilot symbols, and the white dots represent data symbols. In this example, the same symbol arrangement appears at one cycle in four OFDM symbols. In the example shown in FIG. 5, if a delay profile is generated by using four successive OFDM symbols or more, the subcarrier interval at which the pilot symbols are arrayed is three subcarriers, and the maximum time length of the delay profile that can be computed is ⅓ of the effective OFDM symbol length.
FIG. 6A shows a received OFDM signal, and FIG. 6B shows the delay profile generated from the OFDM signal shown in FIG. 6A. In the example described above, the time length of the delay profile that can be computed is ⅓ of the OFDM symbol, and output that exists in the time region other than the ⅓ is unnecessary path information that is generated by repeated cycles. The solid arrows show correct path information, and the dashed arrows show unnecessary path information.
In conventional techniques, in generating a delay profile, the time position (see FIG. 6C) of a delay profile window W (the window showing the time position for extracting the needed path information) is determined from two time positions which are the time position of the FFT window at the time of OFDM demodulating the received signal (the window expressing the time position of the FFT computation) and a reference time position of the delay profile window W.
Note that the time position of the FFT window is determined by using, as a reference, a time position determined by a correlator. The reference time position of the delay profile window W is determined fixedly from the time position of the delay profile that is assumed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2006-352314). Because the reference time position of the delay profile window W is fixed in this way, it can be said that, conventionally, the time position of the delay profile window W is determined in accordance with the results of the correlator.
As shown in FIG. 7A through FIG. 7C, a conventional correlator determines the correlation value (see FIG. 7C) between the received OFDM signal (see FIG. 7A) and the signal (see FIG. 7B) that is that OFDM signal delayed by the effective symbol time length. Then, the maximum value of the value integrating the correlation value is extracted, the time position of that maximum value is detected, and the time position of the FFT window is determined by using that time position as a reference.
When multipath arises, there are cases in which the time position of the FFT window that the correlator detects is not an appropriate position. For example, at the time of receipt of a two-wave multipath, a correlation output such as shown in FIG. 8C, in which the respective correlation outputs shown in FIG. 8A and FIG. 8B are combined, is obtained. If the correlation output has plural peaks in this way, time synchronization is unstable, and there are cases in which the time position of the FFT window that the correlator detects is extremely close to either one of the two arrival waves, and there are cases in which the other path component does not appear correctly in the delay profile.
Specifics will be described with reference to FIG. 9A through FIG. 9F. FIG. 9A and FIG. 9B show the two arrival waves that are received. FIG. 9C and FIG. 9D show the delay profiles generated from FIG. 9A and FIG. 9B, respectively.
In the same way as in FIG. 6B, FIGS. 9A through 9F also include unnecessary path information. However, if the delay profile window W is made to be the appropriate time position, as shown in FIG. 9E, the path information of the two arrival waves can be extracted, and correct delay profile information can be generated. However, in a case in which the time position of the FFT window that the correlator detects is not appropriate, the delay profile window W also is not set at the appropriate position, and, as shown in FIG. 9F, unnecessary path information is taken-in and an incorrect delay profile is generated.
When the time position of the delay profile window W is determined by using correlator output in this way, there are cases in which a good delay profile cannot be generated.